CA2164785A1

CA2164785A1 - Oxide remover

Info

Publication number: CA2164785A1
Application number: CA002164785A
Authority: CA
Inventors: Erich Rollin; Hansjorg Huber; Jean-Pierre Gabathuler
Original assignee: Alusuisse Lonza Services Ltd
Current assignee: 3A Composites International AG
Priority date: 1994-12-22
Filing date: 1995-12-08
Publication date: 1996-06-23
Also published as: CH688613A5; EP0718059A1; EP0718059B1; ATE168296T1; ES2121327T3; DE59502820D1; US5730201A

Abstract

Process for manufacturing shaped parts out of thixotropic metal billets in horizontal pressure diecasting machines such that inclusions of oxide skin surrounding the thixotropic metal billet are avoided in the alloy structure of the shaped part. Before introducing the thixtropic metal alloy into the space inside the mould, the oxide skin surrounding the metal billet is removed completely and collected in a container such that removal of oxide-free, homogeneous thixotropic metal alloy is minimised by taking into account the thermal and mechanical properties of the thixotropic billet, which are asymmetric with respect to the longitudinal axis of the metal billet.

The removal of the oxide skin takes place in a horizontal diecasting machine which contains an oxide remover (30) between the casting chamber (10) and the mould (70). The oxide remover (30) is represented by a ring-shaped body with a horizontal concentric middle axis (m) and a throughput opening. The oxide remover (30) contains a ring-shaped recess, the oxide deposit ring (40) which is connected to the throughput opening (31) of the oxide remover (30) via a concentric, ring-shaped oxide remover opening (42), the cross-section of the oxide remover opening (42) being asymmetric with respect to the concentric middle axis (m) of the oxide remover (30).

Description

-1- d~

O~ide Remover The present invention relates to a process for mAmlf~cturing shaped parts out of thixotropic metal billets in horizontal pressure diecasting machines such that inclusions of the oxide skin 5 surrounding the Il~ixolrùpic metal billet are avoided in the alloy structure of the shaped part.
The invention also relates to a ~liecA~l;ng mA~hine specially developed for carrying out the process accoldillg to the invention.

The process for mAmlfActuring shaped parts out of thixotropic i.e. partially solid/partially 10 liquid metal billets is known as thixofo~ g. Metal billets that come into question for the process are all billets that can be converted to a thixotropic state. In particular, these metal billets may be of ~lllminillm, m~nesillm or zinc and alloys of these metals.

The thixofollllillg of Ihixollùp c metal alloys is known. It is a process in which the 15 thixotropic propel lies of partially solid and partially liquid metal alloys are exploited. In the following text the equivalent e,.~le~ion of semi-solid state is also used for the partially solid/partially liquid state i.e. thixotropic condition of metal alloys. By the thixotropic behaviour of a metal alloy is u ndel~lood that, when no force is applied, such a metal behaves like a solid body; if subjected to shear forces, however, its viscosity is reduced to such an 20 extent that it behaves in a manner similar to that of liquid metal. To achieve this, it is necess~y to heat the alloy up to the t~lllpclal~lre zone for solidification bel~neen the liquidus and solidus telllpelalllres. The telllpelal~lre has to be adjusted such that e.g. a fraction of 20 to 80 wt % of the structure is molten while the rest remains in the solid state.
25 In the thixofolllllng process partially solid/partially liquid metal is ll~l~ulllled into shaped parts in a modified pressure diec~ g m~chine. The pressure diec~ting m~hines employed for thixocasting differ from those used for diec~ting molten metal e.g. in that the former feature a longer casting ch&l.b~r to accommodate the thixotropic metal billet and conseq~l~ntly a longer piston stroke and, for eAall,pl~, ~neçl~ cal reil~rcelllelll of the parts 30 of the die-casting ,~s.-~hil~e that contact the thi-xotropic metal alloy, this because of the higher pleS~ule on these parts during Illlxo~lllling.

TlfiAofolllf,ng is normally carried out using a holizonlal pressure diec~ctin~ m~chine. In theses machines the casting challlber that accommodates the Ih;Aollop:c metal billet lies 35 horizontal, and is ~l ~lged at a right angle to the dividing plane of the mould, i.e. to the front face of the mould with the metal inlet opening. In the ILlAofc.llllll~g process a IhiAollopic 216~8~
metal billet is loaded into such a horizontal casting chamber of a pressure diecAeting ch~llber and introduced into the mould which is normally made of steel, in particular hot working steel, by applying pressure via a piston at high speed and at high pressure i.e. introduced and enclosed in the space inside the mould where the metal alloy solidifies.
s The as-cast structure formed during the solidification of the thixotropic metal alloy in the mould essçnti~lly determines the plOpel lies of the shaped part. The structure is characterised by phases such as parent metal and eutectic phases, the grain structure such as globulitic grains and dendrites, segregalion and also defects in the structure such as porosity (gas 10 pores, microvoids) and hllpulilies such as e.g. oxides.

The semi-solid metal billets employed for thixofolllllng exhibit a fine grain structure which -provided no grain coarsening takes place during the pre-lre~ 1 of the ~IL~ollop.c metal billet i.e. during heating up and II~ISPGI lalion of the metal billet to the pressure diec~cting 15 m~.hine - appears again in the alloy structure of the shaped part. Fine grain improves the material plop~;,lies in general, inCl~SCS the homogeneity of the alloy structure and helps to avoid defects in the structure of the shaped part. TLi~ofolln.ng semi-solid alloys features other significant advantages over pres~ure diec~eting of molten metal. One such advantage is the significant savings in energy and the shorter production times, this because the metal 20 billets, in contrast to liecA~ g molten metal, have to be heated to lower telllpel~ res and therefore undergo shorter heating up times, and also cool faster in the mould i.e. are returned to the solid state, which collllil~ules to a reduction in grain growth. The energy saving is achieved mainly by the fact that a large fraction of the heat of fusion and all of the energy required for o~elhealing is not required i.e. the additional heat supplied to the metal alloy to 25 reach a telllpe~alllre above the melting point in order to arrive at a molten state and the energy for keeping the melt hot is not necess~. A further advantage is the better dimen-sional tolerances due to the smaller amount of shrinkage and the production of parts that are closer to the final dimeneions~ as a result of which machining steps are reduced and alloy material is saved. Also, because of the applo~ çly 100C lower m~mlfActllring 30 telllpelalllre, the individual co---polle.lls of the pressure diecA~ g mAchine are subjected to lower stresses due to thermal cycling, which means a longer service life of tooling. The lower processing tempelalule in ~l~orc.lll~ng, co--.p~ed with molten metal pressure diecAeting makes it possible to process alloys with a low iron content as this element is not taken up from the tooling by the alloy. Furthermore, IL~xofol~llllg enables better filling of the mould 35 along with less enl-~...enl of air.

20s3 ~3 -2164785 On coming into Gontact with the surrounding atmosphere, items made e.g. of ~ minillm, magnesium or zinc, or their alloys acquire a natural oxide skin, the thickness of which is normally far below one micron. As a metal billet is heated up in order e.g. to ~ srol"~ it to a thixotropic state, this natural oxide layer, the so called oxide skin which is usually already 5 present on the outside of the metal billet, becomes thicker. The thickness of the oxide skin formed during the heating up process depends on the heating up time required, on the atmosphere surrounding the billet and on the alloy composition of the billet in question. The thickness of the oxide skin formed during the heating process is, for ~ll,.,,;l~i,,.,, billets, typically 0.1 to 10 llm. Especially in the case of metal alloys in the molten or thixotropic state 10 impurities such as e.g. alkali and alkaline earth metals can gather in the oxide.

The oxides i.e. parts or particles of the oxide skin formed during heating are normally to be found again in the shaped parts. The oxide particles in the thixotropic metal alloy usually form e.g. oxide inclusions in the shaped part or lead to pores being formed in the alloy 15 structure. Further, oxides and other non-met~llic inclusions in the oxide skin can cause separation of the structure of the product. Consequently, the oxide skin on the surface of the thixotropic metal billet impair the quality of the shaped part and, as a result, its mechanical propcl lies.

- 20 Oxide inclusions are lhclcr~ e undesirable, especially in highly stressed parts, or may even prevent parts from being used as components that are exposed to high l~erl-~-ic~l stresses.

A major problem in thixorolllfing thixotropic metal alloys is thereforc the formation of oxide during ple~Le~1"..?~ such as heating up or ~ spollation of the metal billet through the 25 surrounding atmosphere. The llLckness of the oxide skin formed can be reduced, however not totally avoided, by taking special measures such as e.g. enclosing the metal billets in an inert gas. Also, the measures to be taken to reduce the thickness of oxide skin, especially when producing on an ill~lu~ll;al scale, are involved and cA~ensi~e.

30 In view of theses difficulties in thixofoll.ling, the object of the present invention to minimi~e~
at favourable cost, the structural defects in shaped parts caused by oxide inclusions and with that to provide a process for IhiAofolll~ g which avoids inclusion of components of the oxide skin in the shaped parts.

35 That objective is achieved by way of the invention ch~clclised in that, before introducing the thixotropic metal alloy into the space inside the mould, the oxide skin surrounding the 216478~

-metal billet is removed completely and collected in a container and such that removal of oxide-free, homogeneous thixotropic metal alloy is minimised by taking into account the thermal and lllechanical propellies of the LlliAUllopic billet, which are asynllll~llic with respect to the longitu~lin~l axis of the metal billet.

Consequently, shaped parts m~nllf~ctured using the process according to the invention exhibit no oxide inclusions or only a small amount thereof, which is sub-critical with respect to the application intended for the shaped parts.

10 The amount of metal alloy required for c~lying out the process for m~mlf~cturing the shaped part according to the invention is usefully present in billet form. The metal billets are cylindrical in shape and as a rule are round or oval in cross-section; they may, however, also be polygonal in cross-section. The ~i~meter ofthe metal billet is for e~alllpl~ 50 to 180 mm.
usefully 75 to 150 mm and prerelubly 100 to 150 mm. The length ofthe metal billet is for 15 example 80 to 500 mm.

Metal alloys that come into question for the process according to the invention are all col,llllelGially available metal alloys that can be converted to the thixotropic state. The process according to the invention is especially suitable for alloys of All~ ;ni"~", magnesium 20 or zinc. Especially prerelled are Al~ ,." casting and Ahlminillm wrought alloys. The process according to the invention is, to advantage, also suitable for processing pa-rticle reh~l~ed Alllminillm alloys conlA;~ g e.g. homogeneously distributed SiC or Al2O3 particles. The process accordillg to the invention is very specially suitable for Alllminillm alloys that exhibit a large solidification interval such as e.g. AlSi7Mg.
The alloy of the metal billet necessal~ for the process accolding to the invention collLaills e.g.
homogeneously distributed, plhll~y particles from the solidification process, viz., from individual degel~l~ted dendliles. Usefully, the fraction of plhlla,y solidified particles amounts to 40 wt % or more. In the case of ~lllminillm alloys for e~llple, it is necessd,y for 30 the alpha phase to be present in globulitic form to achieve good lLiAollopic behaviour so that the melt and solid fraction flow unirollnly.

The degenerated dendrites in general plert;lubl~ exhibit a globulitic form which enables uniform, homogeneous flow of melt and solid fraction to be achieved without precipitation 35 taking place. A structure e,.l~iling globulitic dendliles is obtahled by methods which include a continuous casting process conlbined with intensive elc~,llo",Agnetic stirring during 5 21647~
`_ solidification. This leads to melting and fracturing of dendrites whiGh become rounded at temperatures close to the solidus, resulting in the globulitic structure.

For thermoforming purposes, the metal billet required for the process according to the 5 invention is initially heated to a telllpelal-lre which is above the solidus te~ )e~ re and below the liquidus telll;)el~ re i.e. until a semi-solid thixotropic condition is reached.

The heating of the metal billets usually takes place in a separate furnace, which may be heated using combustion materials such as e.g. gas or oil, or electrical energy e.g. by 10 resistance or inductive he~ting For the process accol-lhlg to the invention the metal billets are plerel~bly heated in an induction heated furnace. The heating of the metal billets is very important as the state of the billet i.e. its partial ~Llt;ll~ l is exhibited usually only in a very small telllpel~ re range; long heating up times have to be avoided e.g. because of the formation of a thick oxide skin or grain coarsening. Also, in order to achieve a homogeneous 15 final product, the telllpclal~lre distribution in the thixotropic billet, the so called thixo-blank, should be as homogeneous as possible. For that reason, the conversion of the metal billet to a thixotropic state i.e. the heating of the billet until the desired fraction of alloy is molten, is plerel~bly carried out with the furnace telllpel~ re controlled by sensors.

20 The metal billets may be heated by loading them directly into a furnace, or they may be placed first in a colltainel e.g. a metallic conlAi~-e., pl~;r~.~l" of stainless steel, or in a crucible made of ~ min~ phile or ~ min~-sic. During the heating process the metal billets may be in the vertical or holi~onlal position with respect to their lon~ lin~l axis. If a metal billet is heated in the holi~olllal position, it is housed e.g. in a colllail-e.. When it has 25 been converted to the thixotropic state, the metal billet may be ll~ re~led, e.g. by means of a ~ipping facility, into the casting cha,l~ l of the holi~onlal pressure diec~ting m~çhine and delivered for further processing into a shaped co,llponelll. In this case the metal billet 1 e.llains in the same colllallle during the heating up process and during transportation in the casting ch~lll)el.
If the metal billet is loaded directly into the furnace for conversion to the thixotropic state i.e.
without a colll~er accommodating the billet, then the billet is pl~;rt;l~bly in the vertical position with respect to its lon~hltlin~l axis.

35 In the semi-solid state the thixotropic alloy colltaills a Ihl~c.llopic paste-like structure colllplising the solid, dendritic plilllaly particles retarded in growth in a surrounding matrix -6- ~16418~

of liquid metal. The fraction of solid, primary dendritic particles is usefully chosen such that during heating up, transportation to and in the casting chamber, the thixotropic metal billet does not undergo any significant deÇol,llalion, and there is no significant loss of material, for example due to melt leaking out. The thixotropic, pasty structure contains p-eÇel~bly 40 to 5 80 wt.% of plh~aly particles.

The thixo-blank is then pushed by a piston, eÇr~;~ing a thrusting action at the speed of the piston, through the throughput opening of a plerel~bly ring-shaped body, the so called oxide remover, in which the oxide skin of the thixo-blank is, in accordance with the invention, 10 removed and deposited in a colllail1er. The thixotropic metal alloy prepared in this manner is then passed through the inlet opening to the mould interior. The mould itself comprises normally of a fixed half and a moveable half, each half ~,~iling a recess and the recesses of both mould halves together folllllng the interior of the mould which may be at ambient pressure or be evacu~te~l during the process according to the invention. During the heating 15 up of the metal billet and its conversion to a thixo-blank, an essenti~lly unirullllly thick oxide skin is formed over the whole of the thixotropic metal billet. When thixofol-ning with a holizonlal casting ch~ber, the metal billet is laid holi~.onlal. For reasons relating to the process, the dia~ ,ter of the metal billet is normally smaller than the diameter of the casting cha~llber interior. As the casting chd~llbel interior is normally round or oval in cross-section, 20 the lhi~ullopic metal billet Iying holizolllal in the interior ofthe casting chamber rests on only a small area cOlllpal~d with its total surface area i.e. the thixotropic metal billet makes lllechalLcal and thermal contact with the casting cha~ er wall over only a small area e.g. on its lmdermost side.

25 As the casting ch~,lber is at a lower telllpe~ re than the thixotropic metal billet due to the direct thermal contact at the undermost side of the metal billet, more heat is transferred from the thixotropic metal billet to the casting ch~llber than from the other peliph~ l regions of the billet that make no direct ",~ch~l- cal and thermal contact with the casting cha,llber wall, where heat ll~lsrer to the wall takes place only via convection or thermal radiation.
30 Dependil1g how long the metal billet lies in the casting cl-&llber, its lllechalL~I propel lies i.e.
in particular its partial ~lrell~,lll or the viscosity may become inhomogeneous across the billet cross-section. If the telllp~ re of the billet is initially in the telllpelalllre zone which permits the LLxullopic state, then there is a danger that the surface on which the billet rests falls below the t~,lllpel~ re required for the thixotropic state, and this part of the billet is then 35 difficult to process.

20s3 7 ~ 5 - - -For reasons relating to the process itself, the metal billet always cools faster at the surface on which it rests than the rest of the billet. Consequently, the semi-solid fraction or the viscosity in the region close to the contact area, is normally smaller than in the region close to the rest of the outer surface area. At least the viscosity of the metal alloy in the part of the thixotropic 5 billet near the contact surface is higher than in the rest of the lhixollopic metal billet. The semi-solid fraction in the interior of the thixo-blank does not however exhibit any noticeable variation. This semi-solid fraction coll~;sponds ess~ont~ y to that fraction in the part of the thixo-blank close to the outer areas, the surfaces of which make no direct mechanical or thermal contact with the casting cha,llbel wall. Consequently, optimal removal of the oxide 10 skin according to the present invention requires the asymmetric - with respect to the longit~l~lin~l axis of the metal billet - thermal and meçh~nical propc;l lies of the oxide skin and the region of thixotropic metal alloy close to the oxide skin to be taken into account. By optimal removal in the present text is to be understood removal of the oxide skin without ~imlllt~neously removing any substantial amount of lhi~ollop c alloy which is usable for 15 thixoroln~g. In plincipl~ of course a larger concentric outer volume of the thixotropic metal billet may be removed so that only the core region of the thixo-blank is introduced into the interior of the mould. Even if this thixotropic material removed from the thixo~lll~lg process is recycled, the balance of such a procedure with respect to energy consumption and process costs is unfavourable, especially in the industrial production of such parts.
The inhomogeneous propellies ofthe thixotropic metal billet, depel~ding on the pre-ll~l..,ç~l and time in the casting challlber, are not necessalily limited to the oxide skin. For that reason, in a p-e~elled version of the process accordillg to the invention, that part of the ll~Jllopic metal billet close to the area on which the billet rests in the casting challll~er and hence 25 exhibits a smaller liquid rl ..~iOIl, is also removed during the removal of the oxide skin.

In a prerelled version of the process accordil 8 to the invention the thixotropic metal alloy is led through a ring-shaped body, the so called oxide remover, situated between the casting chamber and the mould as a result of which the oxide skin on the thixotropic metal billet is 30 guided under Illecllal ical flow through an inset collc~i~,llic, ring-shaped opening, the so called oxide remover ope~, the cross-section of which is a~ylllllletlic with respect to the con-centric middle axis of the oxide remover opellilg, and from there into a ring-shaped container, the so called oxide deposit ring.

35 Thereby, the ring-shaped body need not necessarily be a separate coll,pon~;lll of the pressure diecasling m~c~ine i.e. the ring-shaped body may also be an appfop,ialely shaped part of the -wall of the pressure diec~ting machine surrounding the thixotropic metal alloy in the region between the casting challlber and the interior of the mould.

The oxide remover may e.g. be a torus shape in the form of a torus-shaped oxide deposit ring 5 which features a ring-shaped oxide remover opening pointing towards its concenllic middle axis.

As the thickness of the oxide skin on the thixotropic metal billet is essentially coll~l~ll all over, so also is prcrela~l~ the amount of oxide removed over the whole peripheral region of 10 the oxide remover, for which reason the cross-section of the oxide deposit ring is usefully axially ~7yllllllcllic with respect to its middle axis. The cross-sectional shape of the oxide deposit ring is non-essenti~l for the process accolding to the invention and may have any desired shape i.e. an area enclosed by an esse~.l;Ally closed curve having an openil1g that is dirccled towards the col1celltlic middle axis ofthe oxide deposit ring. On the other hand it is 15 ess~nti~l to the invention that, also when removing a con~l~ll amount of oxide over the whole pcliphelal region of the billet, i.e. a layer of consLalll thickness over the whole of the cylindrical, pasty surface region of the ILi~.llopic alloy, the conce.lllic ring-shaped oxide remover opening re~.lules an a~,llllæLlic cross-section with respect to its concentric middle axis, the cross-section of the Openillg - in particular in the lower part of the ho.i~olllal ring-20 shaped oxide remover - being larger e.g. than that of the upper part. This way account is taken e.g. ofthe higher viscosity ofthe metal alloy and oxide skin Ol;~A~ g from that part of the thixotropic metal billet close to the area on which the billet rests in the casting ch~llbel .

25 The cross-section of the opclfing which is as~llllllcllic with respect to the concellllic middle axis (m) is pl crcl~bl~ chosen as a function of the viscosity plopel lies of the thixotropic metal alloy which are a;,~ ic with respect to this concentric middle axis (m), and chosen such that a radially un;rullllly thick layer of oxide skin and ll~ullopic metal alloy close to the oxide skin is removed.
Collespol1dil~ly, Ihclcrole, the removal of a radially ~nir~ llly thick layer of ~ minillm oxide and thixotropic metal alloy which exhibits dirrclcllt viscosity, essenti~lly over the peliphcl~l region of the thixo-blank i.e. the surface region of the pasty thixotropic alloy, is achieved by means of a ring-shaped oxide remover the opcnl.lg cross-section of which is 35 dil~l ellL accol ding to the viscosity. In particular the cross-section of the ope. il~g in the lower part of the ring-shaped oxide remover is made larger in order to take into account the higher 20s3 viscosity of the thixotropic metal ori~in~ting from the region close to the contact surface on which the billet lies and the oxide skin.

In especially critical products the higher viscosity of the thixotropic metal alloy o.;gi~ g 5 close to the contact surface may impair the quality of the alloy. For that reason in a further prerelled process this part of the thixotropic metal alloy is removed along with the oxide skin, i.e. instead of removing a radially ullirollllly thick layer of ~ minillm oxide skin and thixotropic metal alloy, a thicker layer of thixotropic metal alloy is removed in the lower part of the oxide remover than in the upper part thereo With this method more material is guided 10 into the oxide deposit ring at the bottom. For that reason the part of the oxide deposit ring in question exhibits a larger cross-section, as a result of which the oxide deposit ring loses its axial~7y~ ly In the present text the term lower or upper part of the oxide deposit ring is always to be 15 understood the part with lertl~,nce to a ho,~onlal plane through the concentric middle axis of the oxide remover.

Very fast filing of the mould during the thixorollllillg process may cause turbulent flow conditions which lead to the enll aplllenl of gases (air, mould sepa- aling or lubricating agents) 20 in the product. Because of the dilr~ t coefflcients of thermal expansion of the metal alloy and the ellll~pp~ gas, this often prohibits any desired subsequent heat ll~l...ç~l of the part.
Such elltl~pped gases lead to pores in the cast structure. Pore formation can be reduced by evacll~ting the mould and/or by slower filling and extraction of air from the mould. Slower filling i.e. filling of the mould interior, aims at avoiding turbulence in the metal alloy - which 25 necçcsit~tes special control of the rate of advance of the piston applying pressure to the billet.
F.ssenti~l here, for the process according to the invention, is that the removal of the oxide skin takes place continuously throughout the whole duration of the process, so that the material removed per unit time is proportional to the rate of advance of the thixotropic billet.

30 The pressure applied by the piston in order to fill the interior of the mould during the thixorolll.illg process is lh~,lerole chosen such that turbulence in the thixotropic metal alloy, and with that the formation of gas and oxide inclusions in the shaped part is avoided as much as possible, i.e. the pressure applied by the piston is prère ~ly such that the thixotropic metal alloy with its surrounding oxide skin experience laminar flow conditions. The pressure 35 applied by the piston to the thixo-blank is e.g. between 200 and 1500 bar, usefully bt;lween lo- 216418~

500 and 1000 bar. The resultant rate of flow of the pasty thixotropic alloy is e.g. 0.2 to 3 ~nls, usefully 0.3 to 2 m/s.

The high pressures applied during solidification improve metal feeding i.e. in order to ensure 5 that the mould is completely filled and reduce the amount of porosity due to shrinkage i.e. to avoid forrnation of so called microvoids. As the thixotropic metal alloy cools in the mould, the density incleases until the solidification point is reached. Shrinkage during solidification causes a high risk of defect formation and can lead to voids in the structure of the finished part. The deficit in volume due to soli~ific~tion shrinkage may amount to 4 to 7.1%.
10 Co.~ ,nsalion for the solid state contraction that acco-.lpali~s cooling after solidification is provided by the allowance for shrinkage made during production of the mould.

The shaped parts m~mlf~ctllred by the process accoldh~g to the invention typically exhibit a polosily of less than 1 vol.% and an oxide fraction of e.g. 0 to 3 wt.%, prerél~bly 0 to 1 15 wt.%. The process according to the invention, thelèrolè, enables safety parts to be made by thixo-fol~ling, the required high elongation propellies being achieved e.g. by the col..l)inalion of low iron alloys (~ 0.15 wt.% Fe), rapid solidification and avoiding oxide inclusions.

The invention also relates to a horizontal pressure diec~cl;ng m~chine for m~mlf~ctllring 20 shaped parts out of thixotropic metal billets such that inclusions in the oxide skin surrounding the thixotropic metal billet are avoided in the alloy structure of the shaped part, and the holi~olllal pres~le ~liec~cl;~ n~ ;ne features a holi~onlal casting ch~l~l)er with a cylindrical shaped hollow interior to acco~ lodate a thixotropic metal billet, a back-up plate with an opening, and a mould with inlet opening and hollow interior.
This is achieved by way of the invention in that an oxide remover is citl1~ted belween the casting chanlbel and the mould, where the oxide remover leplesenls a ring-shaped body with a hol.~olltdl, conc~ lic middle axis and an outer and inner face, and the cross-section through the inner face of the oxide remover pelpend;c~ r to the concellllic middle axis 30 defines the cross-section of the oxide remover, the oxide remover conlains a ring-shaped recess, the oxide deposit ring, which is co~-nF,eled via a concenllic ring-shaped oxide remover opening to the openin~ in the oxide remover defined by the inner face and by the casing cha.lll,er end face and the mould end face of the oxide remover, where the cross-section of the oxide remover opening is as.~llllll~,llic with respect to the col cellllic middle axis of the 35 oxide remover.

- ll - 21647`8~
-During the process of filling the mould, the oxide remover according to the invention acts as a peeling tool that peels offthe oxide skin on the outside ofthe billet which is in a thixotropic state, and holds this oxide skin back in the oxide deposit ring of the oxide remover. The oxide remover is, thelerore, usefully situated immediately upstream of the shape endowing 5 tool i.e. the mould. In horizontal diec~ting m~çhines the billet conlailler i.e. the casting chamber in which the semi-solid metal billet is laid, is horizontal. The casting cha",l)er comprises essentially a cylindrical shaped body which is delimited by the casting chal"bel walls and is hollow inside, the so called interior of the casting chamber; the part of the casting chamber where the thixotropic metal billet is loaded into the chamber i.e. that end 10 facing away from the mould, is e.g. half-shell shaped, while the end of the casting chal~ er facing the mould is in the shape of a closed cylinder, and the hollow space thus created is e.g.
round, oval or polygonal in cross-section.

The diameter of the casting challlber is usefully equal to 102 to 120%, preferably 103 to 15 115% and, highly pl~r~lled, 103 to 110% ofthe diameter ofthe metal billet so that, after it has been loaded into the casting chal,ll~er, the thixotropic metal billet ess~nti~lly makes mççh~nical and thermal contact with the casting challlbel only at its undermost side.

The mould collll3lises e.g. a fixed mould half and a moveable mould half, each mould half 20 featuring a recess which together form the interior of the mould. The inlet for introducing the lhi~ollopic metal alloy into the mould interior usefully features an op~ iscd cross-section with respect to the filling of the mould, and is normally smaller than the cross-section of the opening in the casting ch&,ll~el on the side facing the mould. As a result of the dirrel~
cross-sections in the di~elelll zones (casting challlber, back-up plate opening, inlet opening, 25 mould interior) through which the thixotropic metal alloy flows, the latter c,~elcises difrele forces on the sul,oul1ding walls along the individual regions of the zones of flow, so that e.g.
the resultant axial ll~n~... ss;on of forces onto the walls of the dirrtlellt colllponellls of the horizontal diec~cting lchine differs. To accommodate a fraction of these axial forces i.e.
forces acting in the direction of flow of the ~ ullopic metal alloy towards the mould, a 30 back-up plate with openillg in it is provide ~et~ ,ll the casting cha,llbel and the mould.

Prior to thixoforming, the metal billets are cut to length according to the specific amount of material required, which is defined by the size of the mould interior, and converted to the thixotropic state in a furnace, p,erelabl~ an induction furnace - the ess~nti~lly cylindrical 35 shaped metal billets being held holi~olltal during the heating up process e.g. in a half-shell shaped, cylindrical col,l~L,l~r. Subsequently, the semi-solid metal billets are ll~l~r~lled, 20s3 ~l~ y ~

-m~nll~lly or by means of a manipulator, and loaded into the horizontal casting chamber. In order to prevent the metal billet from solidifying, the thixotropic metal billet must be srelled relatively quickly for further processing i.e. for example within one minute at most. In order to save energy and costs, the casting challlber is not normally heated, i.e. the 5 metal billet cools down continuously, especially the area on which it rests in the casting chamber.

As the cylindrical shaped thixotropic metal billet usefully exhibits a smaller cross-section than the cylindrical and semi-cylindrical casting cl-allll)el, only a small area of it rests on the cast-10 ing ch~llber wall. Because of the good thermal contact this area provides, i.e. due to directconduction of heat, an inhomogeneous distribution of tell,pel~ re is created in the thixo-tropic metal billet. If the thixotropic metal billet rests on a plurality of small area regions of the casting cl~llber wall distributed over the p~;liph~,ly of the billet, the metal billet - as a result of its own weight eAl~ils better thermal contact at the areas of contact underneath it, 15 so that more heat is leleased from the billet dOwll~alds than upwards. Consequently, the pel iphel~l region of the billet that cools most is the undermost area where it makes contact with the casting chdlnl)el wall. As a result, the billet's thermal and ~ nical axial ~yll~ ly - with respect to its concellllic middle axis - achieved in the furnace is lost, which causes e.g.
the viscosity or the liquid fraction of the thixotropic aUoy to become asylll,llellic with respect 20 to the concellll ic middle axis of the metal billet.

During the heating of the ll~ollop:c metal billet and during transportation to the casting cl~llbe, the natural oxide layer which is normally already present b~collles much thicker as a result of the higher billet telllpelalllre in the semi-solid state and the more reactive billet 25 surface this produces. The introduction of parts of particles of oxide skin into the mould interior usually leads to significant defects in the casting or to the formation of pores there, as a result of which the alloy quality in the shaped part may be strongly illlpailed. Using the pressure tliec~ting m~ine according to the invention the oxide skin around the thixotropic metal billet can be cGln~letely removed by means of an oxide remover ~ te~ between the 30 casting ch&~ er and the mould. Thereby, as little as pos;,i~lc of the thixotropic metal alloy that is usable for the thixo~ollll.n~, process should be removed, which makes it n~cess~y to take into account the asylll~ llic thermal and Illecl~lical pl~,pellies of the thixotropic metal billet i.e. asymmetric with respect to the conc~ltlic longitll-lin~l axis ofthe metal billet.

35 In accordance with the invention the oxide remover comprices a ring-shaped body that features in the interior a conc~lltlic, a, for c~ul~ple torus-shaped recess, the so called oxide -deposit ring. The inner part of the oxide remover or the part of thereof represçnting the throughput opening i.e. the space defined by the inner faces and both end faces of this body exhibits, perpendicular to the end faces, a concentric middle axis - the concentric middle axis of the oxide remover - that usefully coincides with the conce~ ic longitudinal axis of the 5 space inside the casting chamber and in particular with the conce.lllic middle axis of the mould inlet. The cross-section of the throughput openil-g perpendicular to the concellllic middle axis of the oxide remover, the so called throughput cross-section, preferably corres-ponds to the cross-section of the opening in the casting chamber on the side facing the mould i.e. the opening in the cylindrical casting chd~llbel facing the mould.
The end face of the oxide remover on the side facing the casting challlber is normally situated directly on the casting challlber openillg on the side facing the mould. The end face of the oxide remover on the side facing the mould is preferably ~it~ted directly on the outer edge of the inlet leading to the interior of the mould, i.e. the end of the oxide remover on the side 15 facing the mould lies directly on the front side of the mould or on the fixed part thereof facing the oxide remover.

The oxide remover opening comprises a ring-shaped recess on the inner face of the oxide remover. It is plerelubly formed by a recess in the oxide remover on the side facing the 20 mould i.e. it is situated on the end face of the oxide remover on the side facing the mould or on the side facing away from the casting cl~"ber. As a result, a cylindrical mantle-shaped space is formed belween the mould-facing end of the inner face and the oxide remover facing front side of the casting cl a-"~el, so that the space formed by this Openillg between the interior i.e. the throughput opel~ng of the oxide remover and the oxide deposit ring, the so 25 called oxide remover opening, is in the shape of a hollow cylinder. The area cut out of the hollow cylinder by a lon~h1~in~l section through the concellllic middle axis of the oxide remover repres~.lts the cross-section of the oxide remover opening.

For reasons relating to l"echal~ical flow conditions the passage in the oxide remover may 30 open conically outwards towards the end of the oxide remover on the side facing the mould.
The wall ofthe conical h~léllælll makes an acute angle of e.g. 2 to 30, ple~el~bly 5 to 15 and in particular 5 to 10 with the holizolllal part of the inner face of the oxide remover, the details concelllillg angles in the present text always being based on a complete circle of 360.

35 The oxide remover, casting chamber and the mould accol~ g to the invention are usefully of material that can be subjected to high thermal and Illechalic~l loading, for example steel, in 20s3 -14- 216478~

particular heat resistant steel (DIN X 38 CrMoV51), of ceramic materials or out of a steel with a ceramic coating on the surfaces exposed to the ll~ol,opic metal alloy. Plere"ed is that at least the components of the pressure diec~ting m~chine that come into contact with the thixotropic material, in particular the oxide remover, are made of heat resistant steel.

For reasons relating to meçh~nic~l flow, the front side of the mould, on the side facing the oxide remover, prerel~bly features an inlet opening that tapers conically inwards, i.e. the inlet opening, usually leading through the fixed half of the mould, exhibits a strongly enlarging angle at the opening facing the oxide remover i.e. an opening angle that diverges only slightly 10 from a right angle e.g. an angle of 80 to 87.

At the front end of the inlet opellh1g, i.e. where the thixotropic metal alloy, moving ess~nti~lly parallel to the conce~ ic middle axis of the inlet opening, illl~,hlges directly on the interior wall of the e.g. moveable mould half, this part of the mould may be provided with a 15 recess which can accommodate the front side part of the oxide skin of the thixo- blank facing the mould.

In a plerelled version of the pressure diec~cting m~hine accolding to the invention, the oxide remover is sihl~ted in the back-up plate opening, whereby the length of the ring-shaped 20 oxide remover usefully collespol1ds to the thickness of the back-up plate, i.e. the length of the back-up plate opening. Normally, during the ~ orc,lllllng process, all parts of the pres~ule diec~ting m~chine in contact with the thixotropic metal alloy are subjected to large forces acting in the direction of flow of the ll~ollop-c alloy and, as a result of the oxide remover opening and the oxide deposit ring, the oxide remover exhibits a smaller wall 25 thickness for t.~ ple at its end facing the mould than at its end facing the casting cl a"~er.
For that reason, the oxide remover usefully features other means of taking up the forces acting on it in the direction of the mould. This may be achieved e.g. by means of a step on the oxide remover on the side facing the casting ch~lll)el, said step being made such that it eng~es in a groove-shaped recess in the back-up plate, thereby taking up the forces acting 30 parallel to the colu~llt~ic middle axis of the oxide remover in the direction of the mould. The groove-shaped recess and the step are prerel~bly radially ~yll~ lical, i.e. their cross-section pel~Jendicular to the concentric middle axis of the oxide remover is preferably in the form of a circular ring.

35 The oxide deposit ring need, however, not necessslily constitute a separate colllponelll ofthe pressure diec~ting m~hine; i.e. the ring-shaped body may also be a suitably shaped part of the wall of the pressure diec~cting m~çhine surrounding the thixotropic metal alloy in the region between the casting cha~ er and the interior of the mould, or an appl Opl iately shaped end of the casting ch~,lbe, on the side facing the mould. The oxide remover is, however preferably a sepalately m~nllfactured part that can be installed between the mould and the 5 casting ch~"ber.

If the oxide remover is in the form of a separate part of the diec~tin~ m~hin~, then it is usefully positioned between the casting cha",ber and the mould. This way the forces resulting from the pressure on the thixo-billet in the casting chal,~er are llansr~;l,ed in the axial 10 direction onto the oxide remover. In order that the oxide remover is not overloaded, p,erell~d means for accommodating these forces are provided on the back-up plate. This can be achieved e.g. by way of an integral or attached brace on the casting chamber wall and an integral or ~ çhed step in the form of a casting ch~,ll)er ~lignm~nt means for example on the outer region of the casting cha",~er. The casting cha"ll)er wall brace and the casting 15 chal"l~er ~lignm~.nt means are p-ere,ably ring-shaped, i.e. their cross-section perpendicular to the concenl,ic middle axis of the oxide remover is prerel7~bly in the form of a circular ring.

Both the asy"",lc;llic cross-section of oxide remover necessary for optimal removal of oxide skin, i.e. asymmetric with respect to the middle axis of the oxide remover, and the necessa~y 20 optimum design and capacily of the oxide deposit ring, depend on the thi~kness of the oxide skin on the thixotropic metal billet and the size (length, diameter) of the metal billet. In turn, the thickness of the oxide skin depends to a large extent on the alloy composition and on the prehistory of the metal billet. The exact d,l,æns;ons of the Ope.lillg cross-section and the oplilllulll shape and c~;ily of the oxide deposit ring must lhelerore be calculated in 25 advance for the part to be made, and trials must be carried out before production.

The lower part of the oxide remover openillg i.e. Iower with respect to a holi~onlal plane through the concel,l,ic middle axis of the oxide remover, or at least a part thereof, i.e. in a segment of the hollow cylindrical oxide remover ope~ g, prere,~ly exhibits a larger cross-30 section than in the upper part. Thereby, the rest of the opening - except this segn,~nl of the hollow cylindrical openi,lg in the oxide remover - may be constant or become larger continuously or in steps towards the bottom. The cross se~,lion in the lower Iying segment of the hollow cylindrical opening in the oxide remover may likewise be con~ l or become larger continuously or in steps from top to~ards the bottom. The segment of the hollow 35 cylindrical opellil-g in the oxide remover with the larger cross-section concerns escenti~lly that region of the opening in the oxide remover through which flows the oxide skin 16 c~ Zg~
_ origin~ting from, and the thixotropic metal alloy close to, the area on which the metal billet rests. The angle of segment in the hollow, cylindrical, oxide remover opening is preferably between 30 and 70 and in particular between 50 and 65 - with reference to a full circle of 360.

In an especially plcrelled version of oxide remover according to the invention, in a longitll~in~l section running perpendicularly through the concellllic middle axis of the oxide remover, the upper part of the oxide remover openhlg - i.e. upper with respect to a horizontal plane through the concel-~lic middle axis of the oxide remover - exhibits a distance 10 of 0.5 to 4 mm, in particular 1 to 3 mm, bclween the inner face of the oxide remover at the mould end and the end face of the oxide remover on the mould side or the front side of the mould on the side facing the oxide remover.

In a further prefelled version of the oxide remover according to the invention in a 15 lon ihlrlin~l section running pcl~cndicularly through the conc~ ltlic middle axis of the oxide remover, the lower part of the oxide remover opel ing - i.e. Iower with respect to a hol i~onlal plane through the concentric middle axis of the oxide remover - exhibits a distance of 1 to 10 mm, in particular 3 to 6 mm, between the inner face of the oxide remover at the mould end and the end face of the oxide remover on the mould side.
In the case of an oxide remover Opel)~ng on the mould-side end face of the oxide remover, the necessary a~ylllll,~,tlic cross-section according to the invention may be in the form of an apploplidle recess in the front face of the mould on the side facing the oxide remover. The recess is prerel~bly ~it~l~te~l in the lower part of the oxide remover opening - i.e. Iower with 25 respect to the holi~olllal plane through the concenllic middle axis ofthe oxide remover - and such that the cross-section in the lower part of the oxide remover opening i.e. in a segment of the hollow, cylindrical shaped oxide remover opening, is el~larged.

The asyllllllctlic cross-section of opening necGssdly for the uniform removal of oxide skin 30 according to the invention i.e. asyl~llllcllic with respect to the concellllic middle axis of the oxide remover may, accoldi~gly, be achieved by a an oxide remover Opc;nil g that is axially ~yllllllcllic to the concentric middle axis ofthe oxide remover and fedtures a recess according to the invention on the front side of the mould on the side facing the oxide remover. A
collespol1ding recess on the front side of the mould on the side facing the oxide remover 35 may, however, also be provided in addition to an oxide remover opening with a an opening cross-section that is already a~yllllllcllic with respect to the conc~l~llic middle axis of the 20s3 -17- 2164~85 oxide remover and thus enlarge the cross-section in this part of the oxide remover opening or serve to improve feeding of the oxide skin into the appropliate part of the oxide deposit ring.

The recess in the front side of the mould on the side facing the oxide remover may be of any 5 desired shape, in particular a cylindrical shape, whereby cylindrical here means a space described by displ~cen ent of an area enclosed by any desired closed curve. The terrn cylindrical incllldes in particular also parallelepiped, cylinder-segment or hollow-cylinder-segment shaped recess. Further prer~lled shapes are barrel-shaped or blunted-pyramid shaped recesses. The spatial dimensions of the recess are plerel~bly chosen such that the 10 recess il CI eases the cross-section of the oxide remover openlng in the region where the oxide skin close to the area on which the metal billet rests is removed. ~It;f~lled shapes of recess exhibit - in the vertical plane through the concentric rniddle axis of the oxide remover - a maximum height of 10 to 40 mm, in particular 10 to 20 mm and a maximum width of 20 to 80 mm, in particular 20 to 50 mm and, in the direction towards the concellllic middle axis, a 15 maximum depth of 2 to 20 mm, in particular 2 to 8 mm. The recess also preferably exhibits a volume of 0.4 to 64 cm3.

In order to remove an oxide skin of essenti~lly conslanl thickness over the whole of the thixotropic metal alloy, an oxide deposit ring with axially ~yllllllellic cross-section is 20 pr~;rt;lled i.e. axially ~yml~ællic with respect to the concentric middle axis of the oxide remover. The shape of the oxide deposit ring is non-essenti~l The oxide deposit ring may, for example, be a torus-shaped recess in the ring-shaped oxide remover with a ring-shaped oxide remover op~.~ng, wher~y the torus-shaped recess may result from rotation of an area enclosed by any desired closed curve with an opening that is directed towards the axis of 25 rotation around the conc~.ltlic middle axis of the oxide remover. The concenllic middle axis of the torus-shaped recess coincides therefore with the concentric middle axis of the oxide remover. The cross-section of the oxide deposit ring may be rect~n~ r, circular or elliptic in shape. For process control purposes, the ring-shaped oxide deposit ring may additionally be divided by dividing walls into individual regions in order to improve the removal of the oxide 30 skin The capacity of the oxide deposit ring, i.e. the volume of the torus-shaped recess is usefully chosen such that it is at least equal to the volume of oxide skin and any thixotropic metal alloy to be removed at the same time. The capacity of the oxide deposit ring is p[efel~bly 35 between 1 and 10 vol.%, in particular 3 to 6 vol.% of the ll~ollopic metal billet, i.e. of the thixo-blank introduced into the casting chamber.
20s3 ~_ - 18- 2164785 For reasons relating to meGhanical flow behaviour, in order to remove the oxide skin and the thixotropic metal alloy close to the oxide skin, i.e. the material to be removed, a particular thickness is required in order that the material to be removed - for example due to its viscosity and cohesion - is able to flow through the oxide remover openillg. For that reason, 5 in order to achieve continuous uniform removal, the pressure in the pasty thixotropic alloy -at least in the region of the oxide remover opening - should remain constant for a given oxide remover opening. Frequently, however, the pressure in the pasty Ihixullopic alloy with its surrounding oxide skin is not cohslanl or at least not sufficiently COIIS~

10 In a pl~relled version of the holi~onlal diec~ting m~çhine according to the invention the oxide deposit ring comprises a plurality of ring-shaped recesses, the so called oxide deposit ring challlbel~, i.e. instead of only one single torus-shaped recess in the oxide remover, a plurality of torus-shaped recesses is provided, whereby the torus-shaped recesses are interconne~;led by a ring-shaped oxide remover opening. The capacity of the individual oxide 15 deposit ring chambers may thereby be collespondingly smaller than when using a single torus-shaped recess. Especially prerelled is for all the oxide deposit ring cl anlbel~ of an oxide remover to form recesses on the mould side ofthe collespol1ding oxide remover.

Very highly plerelled are oxide deposit ring chall,be,~ with reference to their shape, and 20 individual oxide remover opel~il gs of the oxide deposit ring cha-"be,~, designed such that -with respect to the individual plessu e built up in the ll~ullopic alloy during the thixofo~"i,~ process or accoldil-g to the pressure applied to the thixo-blank - they permit optimal removal of the oxide skin and the llLxoll op.c metal alloy close to the oxide skin.

25 Especially prere"ed is for the oxide deposit ring to exhibit 1 to 5, in particular 3, oxide deposit ring cha."bel~ and a co,lespollding number of oxide removal openll~gs. Thereby, each oxide deposit ring cl~ber, and the optimal cross-section of the collc~JOndillg oxide remover opening for filling this ch~"ber, collespol ds to a phase in which pressure is applied during the thixoro"",ng process - each such phase of pressure application being selected such 30 that, the r esislance to filling - offered by the part of the mould with the coll e~ol ding cross-section of space to be filled throughout that phase - can be overcome.

Normally, it is necess~ry to have various phases of applying pressure in the thixorc.~,ung process. The first phase, e.g. the filling of the mould, takes place under relatively low 35 pressure. Following this, for ~,A~"~,le in order to complete the filling in the edge regions of the mould, the pressure must be increased. The phase at which the highest pl`eS~UI e is applied -in order to prevent microvoids or pores, is during solidification of the part. In this last phase there is no flow of metal forming the part and so no oxide skin has to be removed. In this last phase i.e. the solidification phase, thixotropic metal may flow but the amount of metal flowing is normally so small that it no longer reaches the space in the mould where the part is 5 formed and so is in~ignificant as far as the propel~ies of the part are concerned. As the pressure applied according to the resisl~ce to filling is increased continuously or in steps during the thixorolllling process, the inner ring-shaped oxide removal openings exhibit a larger average cross-section than the outer oxide remover openings.

10 In order to achieve a better quality of alloy in the shaped part, the high viscosity thixotropic metal alloy from the region close to the area on which the billet rests, close to the oxide skin, can be removed along with the oxide skin. For that purpose it is necessaly to provide an oxide deposit ring with a larger capacity in that region. In a further pl~ft:lled form of oxide deposit ring at least a part of the lower part of the oxide ring i.e. lower with respect to a 15 holi~olllal plane through the conce.lllic middle axis of the oxide remover, a larger cross-section than in the upper part, i.e. the oxide deposit ring exhibits a~y,..l..et-y with respect to the concentric middle axis of the oxide remover. A lon~ih~inAl cross-section running vertically through the concenllic middle axis of the oxide remover prefelably exhibits in the lower half of the oxide remover - i.e. Iower with respect to a holizonlal plane through the 20 middle axis - a one to three times, in particular a 1.1 to 1.8 times greater longitudinal cross-sectional area of oxide deposit ring than in the upper half of the oxide remover.

The ho~i~olllal pressure diecA~ g mA~hine according to the invention is in p-il~iple suitable for thixofolll-illg all metal alloys that can be converted to the ll~olropic state and exhibit an 25 oxide skin or form an oxide skin during pre-l-~A.~."~ for c~llple during heating up. The ho.~onlal pressure ~ieGAC~ m~Ghine accoldhlg to the invention is pl~rel~ly employed for ~Li~ofo.ll~g alloys of Alllminillm mAg~ ... or zinc. The horizontal pressure diecActing mA~.hine accoldhlg to the invention is e~ec;ally prertlled and suitable for thixofol..ling alu-minium diecActing alloys, in particular for AlSiMg, AlSiCu, AlMg, AlCuTi and AlCuZnMg 30 alloys The ho.i~onlal pressure diec~c~ mA~hine accolding to the invention permits thert;fore the optimal removal of oxide skin sulloulldillg the thixo-blank shortly before filling the mould and makes it possible therefoLe to produce shaped parts without inclusion of parts of the 35 oxide skin. Furthermore, the ho~i~Gnlal pressure diecAA~ting mA~hine according to the 20s3 -2~- 2164785 invention enables a minim~l loss of material i.e. thixotropic metal alloy that can be used for thixoforming purposes, to be achieved.

The present invention is explained in greater detail by way of l Aal~ les with the aid of figures 5 lto5.

Figure 1 shows part of a vertical, lon~ihl~in~l section running through the middle axis of a hor~ontal diec~tin,~ m~t~.hine.

10 Figure 2 shows a view of a vertical, longihldin~l section running through the concentric middle axis of an oxide remover according to the invention, the oxide remover in figure 2a showing an oxide remover with an oxide deposit ring Iying axially ~ymmellical to the concentric middle axis of the oxide remover and figure 2b showing an oxide remover with oxide deposit ring Iying asyllllnell;cal to the concentric middle axis.
Figure 3 shows a vertical lon~ lin~l section through the concentric middle axis of an oxide remover Iying against the fixed half of a mould and, at right angles to the concentric middle axis, a cross-section along the line A-A through the front side of the mould on the side towards the oxide remover.
Figure 4 shows a vertical longihl~in~l section through the conc~"tlic middle axis of an oxide remover Iying against the fixed half of a mould, whose oxide deposit ring exhibits three oxide deposit ring challlbel~ and three oxide remover openings for these.

25 Figure 5 shows by way of eA~ pl~ a plot of the pressure p arising in the IhiAollopic metal alloy as a function of time during the IhiAofol llfillg process.

Figure 1 shows, by way of CA~IIPIC~ part of a vertical, longitlltlin~l section running through the middle axis of a holi~olllal diec~ g m~chine~ revealing the oxide remover part of the 30 holiGonlal casting cl~l)el 10, the oxide remover 30, the back-up plate 20 and the mould 70.
The oxide remover 30 lies within the back-up plate Opellillg 24, i.e. between the casting cha~ eL 10 and the mould 70.

The casting chall,l)el 10 features a space 11 which is enclosed by a cylindrical casting 35 cl~,lbel wall 12 and serves to accommodate the IhiAollop-c metal billet which is not shown here. The casting cha-llber space 11 leprese.lts esse ~ ly a cylindrical shaped body which is -21- 216478!~
-delimited by the wall 12 The casting chamber 10 is however surrounded by a closed cylindrical mantel, the casting cha~ er wall 12, only in the region of the casting chal,.l)er opening 13 on the side facing the mould and, on the side facing away from the mould, features a shell in the form of a half-cylinder, which is not shown here and is used for loading 5 the thixotropic metal billet The hollow casting cha.llbel space 11 created by the walls 12 features for e,~al-llJle a round, oval or polygonal cross-section In the region of the casting chamber opening 13 on the side facing the mould the casting cha llbel 10 is thel~-e in the form of a hollow cylinder The diameter of the casting challlber space 11 corresponds prefer-ably to 103 to 115% of the ~i~meter of the ll~ollopic metal billet, so that after it has been 10 introduced into the casting clalllber 10, the metal billet makes lllecl-allical and thermal contact with the casting ch~ll~el wall 12 only at its undermost part During the thixofo..,.illg process, a piston - not shown here - which is introduced into the end of the casting chall.ber 10 facing away from the mould 70, presses the ll..~ollop.c alloy under high pressure into the space 68 inside the mould 70. During the pressure diec~ting process, the thixotropic metal 15 billet is initially l-~1sp~.led at high speed in the hollow cylindrical part ofthe casting ch~,-l)er 10 whereby, at the latest after the metal billet meets the front side 46 of the mould 70 on the side of the oxide scraper, the thixotropic metal billet or thixo-blank loses its original shape and e g in the region of the casting cha---ber openil1g 13 fills the whole of the casting chs~--ber space 11 The mould 70 shown in figure 1 co.n~,-ises a fixed mould half 50 and a moveable mould half 60, each mould half 50, 60 featuring a recess 54, 66 which together form the interior 68 of the mould 70. The inlet 52 for introducing the Ll i~ol-op;c metal alloy into the interior 68 of the mould 70 usefully features an opli.. iscd cross-section with respect to the filling of the 25 mould, and is smaller than then cross-section of the Opel)~l~g 13 in the casting chdlnl)el on the side facing the mould For reasons relating to metal flow behaviour, the front side 46 of the mould 70 facing the oxide remover exhibits a part 56 that tapers conically i..~dlds towards the inlet Opellll~g 52 30 i e the inlet openil-g passing through the fixed half 50 of the mould 70 exhibits, on the part 56 of the inlet opelling 52 on the side facing the oxide remover, a pronouncedly enl~g ng openil-g angle i e an openil1g angle that deviates only slightly from a right angle At the front end of the inlet opening 52, i e where the thixotropic metal alloy running 35 essPnti~lly parallel to the concentric middle axis m of the inlet opening 52 strikes directly -22- 216~ 85 against the wall of the mould space 66 in the moveable half 60 of the mould 70, there is also a recess 64 which can accommodate the oxide skin on the front side of the thixo-blank.

The oxide remover 30 is in the form of a ring-shaped body that fe~lules a concentric, ring-5 shaped, e.g. torus-shaped recess, the so called oxide deposit ring 40 on the inside. The inner part of the oxide remover 30 i.e. the space defined by the inner face 36 and both end faces 37,38 of this body i.e. the throughput opening 31 of the oxide remover 30, features vertical to the end faces 37, 38 a conce~ ic middle axis - the concentric middle axis m of the oxide scraper 30 - which is coincid~nt with the concentric longit~l~lin~l axis of the casting challlbel 10 space 11 and with the concentric middle axis of the inlet opening 52. The cross-section of the throughput opening 31 vertical to the concenllic middle axis m of the oxide remover 30, the so called throughput cross-section, corresponds to the cross-section of the casting cha...ber 13 on the mould side, i.e. to the opening in the cylindrical casting ch~llber 10 facing the mould.
The end face 38 on the casting cha-llber side of the oxide remover 30 is situated directly on ope-~ing 13 of the casting cha llbel on the side facing the mould. The end face 37 of the oxide remover on the side facing the mould is situated directly at the outermost edge of the opening 52 leading to the interior of the mould or on its conical inlet opening 56 i.e. side 37 20 of the oxide remover 30 facing the mould lies directly on the front side 46 of the mould that faces the oxide remover 30 or on the fixed half 50 of the mould.

The oxide remover opening 42 is represented by a ring-shaped recess in the inner face 36 of the oxide remover 30. It is formed by a recess in the oxide remover 30 i.e. it is situated on 25 the end face 37 on the side facing the mould or on the side of the oxide remover 30 facing away from the casting ch~llbel 10. As a result a cylindrical shaped space is provided bt;lween the end of the inner face 36 on the side facing the mould and the front side 46 of the mould 70 on the side facing the oxide remover 30, so that the space provided, i.e. the oxide remover op~)ing 42, created by this Opeliing between the throughput Opel ing 31 of the oxide remover 30 30 and the oxide deposit ring 40, is in the form of a hollow cylinder. The area of the hollow cylinder formed by a lon~ lin~l section through the conce.ltlic middle axis m of the oxide remover 30 reples~,.lts the cross-sectional Op~il g of the oxide remover opcnll~ 42. The throughput open.ng 31 ofthe oxide remover 30 il~CleaSe~S COliCally to~d~ds the end face 37 of the oxide remover on the side facing the mould, as a result of which a conical inclemenl 35 34 is formed. Shown in the lower part of the fixed mould half 50 in figure 1 is also a recess 44 which h~creases the cross-section of the oxide re.nover opening 42 in that region.
20s3 - 23 ~ 21~47~5 -In order to accommodate the forces acting in the axial direction i.e. in the direction of flow of the thixotropic metal alloy towards the mould 70, a back-up plate 20 with opel~ing 24 is provided between the casting challlber 10 and the mould 70. The oxide remover 30 is situated in the back-up plate opening 24, the length of the ring-shaped oxide remover 30 5 collesponds to the thickness of the back-up plate 20 i.e. the length of the back-up plate Opening 24. Normally, during the thixofolll"ng process, high forces acting in the direction of flow of the thixotropic metal alloy arise in all parts 12, 30, 70 of the diec~cting m~rhine that contact and guide the thixotropic alloy. Also, as a result of the oxide remover opening 42 and the oxide deposit ring 40, the oxide remover 30 exhibits a smaller wall thickness in that 10 region than at its end 38 facing the casting cha-llbel. For that reason, the oxide remover 30 exhibits, on the side 38 facing the casting cl~lll ~;l, an integral step 32 which is de~igned such that it eng~ges in a groove shaped recess 22 in the back-up plate 20, thus taking up a some of the forces acting parallel to the concen~lic middle axis m of the oxide remover in the direction of the mould 70. The groove shaped recess 22 and the integral step 32 are radially 15 ~yllllllellical i.e. their cross-section pel~eu~dicular to the concellllic middle axis m of the oxide remover 30 is circular in shape.

In order to reduce the ll~lsrer of axial forces from the casting chamber 10 to the oxide remover 30, a casting challlber mantel 16 is provided either as an integral collll)onelll or 20 mounted thereon. A step in the form of an Alignment means 14 for the casting ch&lllber is provided ~tt~ched to, or as an integral part of, the casting chamber 10 at its outermost peliphel~l region and a ~ ce from the mould-facing end of the casting challlbe opening 13 collè~ponding to the length of the mantel 16. The casting ch~llber mantel 16 and the casting ch~l,bel ~lignment means 14 are ring-shaped i.e. their cross-section pel~,~ndicular to 25 the conce-lllic middle axis m of the oxide remover 30 is circular. The inner ~i~meter of the ring-shaped casting chall,ber mantel 16 co~ sponds essenti~lly to the outer diameter of the casting chamber wall 12, and the outer ~ m~o~ter of the casting cha",beL ~lignment means 14 is larger than the inner diameter of the casting chal"ber mantel 16.

30 Figure 2a shows a view of a vertical longit~1-1in~1 cross-section through the concentric middle axis m of the oxide remover according to the invention, where the oxide remover 30 features an oxide deposit ring 40 lying axially ~yllllllellical to the COllCelllliC middle axis m. The lower part of the oxide remover opening 42, i.e. Iower with respect to a holi~onlal plane through the concenllic middle axis m, fealu es - at least in a part thereof - i.e. in a segment of the 35 hollow cylinder shaped Opelling 42 in the oxide remover 30, a larger cross-section of opening than in the upper part. Up to this segment of the hollow cylinder shaped opening, the cross-section of the opening may be constant or be enlarged either stepwise or continuously towards the bottom. The cross-section of the opening in the lower Iying segment of the hollow cylinder-shaped oxide remover opening 42 with the larger cross-section may likewise be constant or increase continuously or in steps downwards. As such the segment of the 5 hollow cylinder-shaped oxide remover opening 42 with the larger cross-section concerns es~e.nti~lly the region of the opening 42 through which flows the part of the oxide skin and the thixotropic metal alloy origin~ting from the region close to the areas on which the thixotropic metal alloy rests.

10 The throughput opening 31 is enl~,ed rO,l,l,-~g a conical increment 34 towards the end face 37 of the oxide remover 30 facing the mould and allows the oxide skin and the metal alloy Iying close to the oxide skin to flow better into the oxide deposit ring.

The oxide deposit ring 40 shown in figure 2a is axially symmetric to the concentric middle 15 axis m of the oxide remover and is suitable for removing an oxide skin of essenti~lly constant thickness over the whole peli~ y of the thixotropic metal alloy. The cross-section of oxide deposit ring shown in figure 2a displays particularly good properties with respect to guiding the oxide skin and the thixotropic metal alloy close to the oxide skin.

20 Figure 2a shows a view of a vertical lon~t~l~in~l cross-section through the concentric middle axis m of the oxide remover 30 acco-ding to the invention, where the oxide remover 30 features an oxide deposit ring 40 Iying axially a~y"",.~ ical to the concenllic middle axis 30.
The lower part of the oxide deposit ring 40, i.e. Iower with respect to a horizontal plane through the conce,ll,ic middle axis m of the oxide remover, reaIures - at least in a part 25 thereof- a larger cross-section than in the upper part, i.e. the oxide deposit ring 40 exhibits asy",l"G~,y with respect to the concG,lt~ic middle axis m of the oxide remover 30. An oxide deposit ring 40 of such a design, with an enlarged cdpacily in the lower part is particularly suitable for removing thixotropic metal alloy close to the oxide skin from the region close to the area on which it rests i.e. a region that exhibits higher viscosity than ll,i~olropic metal 30 alloy in the interior.

The longit l~in~l section through the oxide remover 30 shown in figure 2b also shows a cross-section of oxide deposit ring 40 which is particularly suitable for guiding the material that has been removed. The oxide remover opelling 42 has the same shape as that of the 35 oxide remover 30 shown in figure 2a.

-25- 216~785 Figure 3 shows a vertical longitll~1in~1 cross-section through the concentric middle axis m of an oxide remover 30 on a fixed half 50 of a mould 70 and a cross-section - along A - A at right angles to the concell~lic middle axis m - through the front side 46 of the mould 70 on the side facing the oxide remover 30. The recess 44 is preferably situated in the lower part of 5 the oxide remover opening 42. i.e. Iower with respect to the holizol~tal plane through the concentric middle axis of the oxide remover, and is arranged such that the cross-section of the opening in this lower region of the oxide remover opening 42 i.e. a segment of the hollow cylinder-shaped oxide remover opening 42, is enlarged. The recess 44 on the front side 46 of the mould 70 on the side facing the oxide remover 30 is provided in addition to an oxide 10 remover opening 42 with a cross-section that is already asymmetric with respect to the concentric middle axis m of the oxide remover 30, and therefole enlarges the cross-section in this part of the oxide remover openillg 42, and serves to provide better guidance of the oxide skin into the corresponding part of the oxide deposit ring 40.

15 The plan view shown in section A - A of the front side 46 of the mould 70 on the side facing the oxide remover 30 and the fixed mould half 50 shows in particular a prerelled design of the recess 44 and its position with respect to the oxide remover opening 42 and the inlet 52.
The recess 44 shown along line A - A in figure 3 concerns a seglll~ of the oxide remover opel~ing 42 which encloses an angle of about 65.
Figure 4 shows a vertical long~tu-linal cross-section through the concentric middle axis m of an oxide remover 30 resting against a fixed half 50 of a mould 70, the oxide deposit ring 40 of which exhibits three oxide deposit ring cha.llbels 40a, 40b, 40c and the related oxide remover openings 42a, 42b, 42c, where the oxide deposit ring chambers 40a, 40b, 40c with 25 respect to their capacity, and the oxide remover openillgs 42a, 42b, 42c with respect to their cross-section, are de~i~ed such that they permit optimal removal with respect to the pressure (p) arising in the thixotropic alloy during the thixorcllllillg process, - i.e. continuous and uniform removal over the whole pel;p~l~l region of thixotropic semi-solid alloy - of oxide skin and the thixotropic metal alloy close to the skin The oxide deposit ring ch&lllbel~ 40a, 40b, 40c are interco~ ected via their respective oxide remover openings 42a, 42b, 42c, i.e. an oxide remover opel~h~g 42a to join the throughput opening 31 of the oxide remover 30 to the oxide deposit ring 40a, an oxide remover opening 42b to join the oxide deposit ring 40a to the oxide deposit ring 40b and an oxide remover 35 opening 42c to join the oxide deposit ring 40b to the oxide deposit ring 40c. The oxide remover openlllgs 42a, 42b, 42c are selected such that they permit continuous removal of the 20s3 -- -26 216~785 material to be removed during the three phases of the thixoro~ ,ng process. Consequently, the ring-shaped oxide remover openings 42a, 42b, 42c exhibit an average cross-section that becomes smaller from inside towards the outside i.e. the average cross-section of the opening 42a is larger than the average cross-section of opel~ing 42b, which is larger than the 5 average cross-section of openillg 42c. By average cross-section of opening is meant here the cross-section taken as an average over the length of opening.

Figure 5 shows a sch~m~tic7 diag,an"l,alic cA~ ple of the pressure p arising as a function of time t in the thixotropic metal alloy during the thixofo~ ng process. In the first phase of the 10 thixoru~ g process, up to time tl in the process, i.e. during the passage of the thixotropic, semi-solid alloy through the inlet openlllg and during the filling of the large-volume region of the mould space 68 bordering on the inlet opel~ng 52, only a small pressure p~ is built up in the semi-solid alloy, so that - in order to enable the material that is to be removed to pass through the oxide remover ope~ g 42a - the latter opening must be of large cross-section. In 15 a second phase ofthe thixofulllling process, in the time interval between time tl and t2 in the process, during the filling of the small volume space 68 in the mould or the mould regions with small cross-section, in particular regions at the e,~l,t;l",lies of the mould interior 68, the pressure p2 in the thixotropic semi-solid alloy usually rises suddenly, and the pressure applied to the thixo-billet has to be hlc,~ssed accordingly. In keeping with the higher pressure, the 20 cross-section of the oxide remover openillg 42b must be chosen such that it is smaller than that of 42b in order to ensure continuous and IJnlrollll removal of material. In a third phase of the thixof~"~",ng process, in the time interval b~weell t2 and t3 in the process, the pressure applied to the thixo-billet is incleased further to a pressure p3, this in order to fill e.g. regions of the mould interior 68 that are very small in cross-section - which for the third phase 25 neces~it~tes an oxide remover Opelllng 42c of still smaller cross-section than the cross-section of opening required for the second phase. Thereafter, the pressure applied is normally hlcleased further in order to prevent e.g. microvoids or pores from rc""~lg during the solidification stage. During the third phase of the thi-xoro"",llg process, however, no further thixotropic material contributing to the product flows into the mould interior 68, so that 30 during this phase oxide skin and lLixollopic alloy close to the oxide skin need no longer be removed.

Claims

1. Process for manufacturing shaped parts out of thixotropic metal billets in horizontal pressure diecasting machines such that inclusions of the oxide skin surrounding the thixotropic metal billet are avoided in the alloy structure of the shaped part, characterised in that, before introducing the thixotropic metal alloy into the hollow interior (68) of the mould (70), the oxide skin surrounding the metal billet is removed completely and collected in a container (40) and such that removal of oxide-free, homogeneous thixotropic metal alloy is minimised by taking into account the thermal and mechanical properties of the thixotropic billet, which are asymmetric with respect to the longitudinal axis of the metal billet.

2. Process according to claim 1, characterised in that that part of the metal alloy of the thixotropic metal billet in the casting chamber (10) which originates from the region near the contact surface, and has the smaller liquid fraction due to the stronger cooling, is removed along with the oxide skin.

3. Process according to claim 1, characterised in that the thixotropic metal alloy is led through a ring-shaped body, the oxide remover (30), situated between the castingchamber (10) and the mould (70), as a result of which the oxide skin of the thixotropic metal billet is led mechanically in a stream through a concentric, ring-shaped opening, the oxide remover opening (42), which is set into the oxide remover (30) and has a cross-section that is asymmetrical with respect to the concentric middle axis (m) of the oxide remover opening (42), and into a ring-shaped container, the oxide deposit ring (40).

4. Process according to claim 3, characterised in that the cross-section of the opening, which is asymmetric with respect to the concentric middle axis (m), is selected as a function of the viscosity properties of the thixotropic metal alloy, which are asymmetric with respect to this concentric middle axis (m), in such a manner that a radial uniformly thick layer of oxide skin and the thixotropic metal close to the oxide skin is removed.

5. Process according to claim 3, characterised in that that part of the thixotropic metal alloy in the casting chamber (10) which originates from the part of the metal billet near the contact surface, and has higher viscosity than the rest of the thixotropic semi-solid alloy, is removed along with the oxide skin.

6. Process according to claim 1, characterised in that the removal of the oxide skin takes place continuously throughout the whole thixoforming process, so that theamount of material removed per unit time is proportional to the rate of advance of the thixotropic metal billet.

7. Horizontal pressure diecasting machine for manufacturing shaped parts out of thixotropic metal billets such that inclusions of the oxide skin surrounding thethixotropic metal billet are avoided in the alloy structure of the shaped part, and the horizontal pressure diecasting machine features a horizontal casting chamber(10) with a cylindrical shaped hollow interior (11) to accommodate a thixotropicmetal billet, a back-up plate (20) with opening (24) and a mould (70) with inletopening (52) and hollow interior (68), characterised in that, an oxide remover (30) is situated between the casting chamber (10) and the mould(70), where the oxide remover (30) represents a ring-shaped body with a horizontal concentric middle axis (m) and an outer and inner face (36), and the cross-section through the inner face (36) of the oxide remover (30) perpendicular to the concentric middle axis (m) defines the cross-section of the oxide remover(30),the oxide remover (30) contains a ring-shaped recess, the oxide deposit ring (40) which is connected led via a concentric ring-shaped oxide remover opening (42) to the opening (31) in the oxide remover (30) defined by the inner face (36) and by the casing chamber end face (38) and the mould end face (37) of the oxide remover (30), where the cross-section of the oxide remover opening (42) is asymmetric with respect to the concentric middle axis (m) of the oxide remover (30).

8. Diecasting machine according to claim 7, characterised in that the concentricmiddle axis (m) of the oxide remover (30) is coincident with the concentric middle axis of the metal inlet opening (52) in the mould (70) and the concentric longitudinal axis of the casting chamber space (11).

9. Diecasting machine according to claim 7, characterised in that the cross-section of the opening (31) corresponds to that of the casting chamber opening (13) on themould side.

10. Diecasting machine according to claim 7, characterised in that the ring-shaped oxide remover opening (42) is formed by a recess in the oxide remover (30) on the mould side.

11. Diecasting machine according to claim 7, characterised in that the part of the oxide remover opening (42) which is lower than a horizontal plane through the concentric middle axis (m), at least a part thereof, has a larger opening cross-section than the upper part.

12. Diecasting machine according to claim 7, characterised in that, in a longitudinal section running perpendicular through the concentric middle axis (m) of the oxide remover (30), the upper part of the oxide remover opening (42), i.e. upper with respect to a horizontal plane through the concentric middle axis of the oxide remover, exhibits a distance of 0.5 to 4 mm between the inner face (36) of the oxide remover (30) at the mould end and the end face (37) of the oxide remover (30) on the mould side.

13. Diecasting machine according to claim 7, characterised in that, in a longitudinal section running vertically through the concentric middle axis (m) of the oxide remover (30), the lower part of the oxide remover opening (42), i.e. lower part with respect to a horizontal plane through the concentric middle axis, exhibits a distance of 1 to 10 mm between the inner face (36) of the oxide remover (30) at the mould end and the end face (37) of the oxide remover (30) on the mould side.

14. Diecasting machine according to claim 10, characterised in that, in the case of an oxide remover opening (42) on the mould side face (37) of the oxide remover (30), a recess (44) is provided in the front face (46) of the mould (70) on the side facing the oxide remover (30), such that the cross-section of the oxide remover opening (42), or a part thereof, in the lower part with respect to a horizontal plane through the concentric middle axis (m) of the oxide remover (30), is enlarged.

15. Diecasting machine according to claim 14, characterised in that the recess (44) is cylindrical, barrel or blunted pyramid in shape.

16. Diecasting machine according to claim 14, characterised in that, in the vertical plane through the concentric middle axis (m) of the oxide remover (30), the recess (44) exhibits a maximum height of 10 to 40 mm and a maximum breadth 20 to 80 mm and, in the direction of the middle axis (m) a maximum depth of 2 to 20 mm.

17. Diecasting machine according to claim 7, characterised in that the capacity of the oxide deposit ring (40) comprises 1 to 10 vol. % of the thixotropic metal billet.

18. Diecasting machine according to claim 7, characterised in that the oxide deposit ring (40) comprises a plurality of ring-shaped spaces, the oxide deposit ring chambers (40a, 40b, 40c) with a common concentric middle axis (m) which coincides with the concentric middle axis of the oxide remover (30) and the oxide deposit ring chambers (40a, 40b, 40c) are joined to each other, each via an oxide remover opening (42a, 42b, 42c).

19. Diecasting machine according to claim 18, characterised in that the shape of the oxide deposit ring chambers (40a, 40b, 40c) and the oxide remover openings (42a,42b, 42c) to the individual oxide deposit ring chambers (40a, 40b, 40c) are designed such that they permit, with respect to the pressure (p) arising in the thixotropic alloy during the thixoforming process, optimal removal of the oxide skin and the thixotropic metal alloy close to the skin.

20. Diecasting machine according to claim 19, characterised in that the oxide deposit ring (40) contains 1 to 5 oxide ring chambers (40a, 40b, 40c) and connecting them 1 to 5 ring-shaped oxide remover openings (42a, 42b, 42c).

21. Diecasting machine according to claim 7, characterised in that the part of the oxide deposit ring (40) lower than a horizontal plane through the concentric middle axis (m) of the oxide remover (30), at least a part thereof, has a larger cross-section than the upper part.

22. Diecasting machine according to claim 21, characterised in that a vertical longitudinal section through the concentric middle axis (m) of the oxide remover(30) in the lower half- with respect to a horizontal plane through the concentric middle axis (m) - of the oxide remover (30) exhibits a one to three times largerlongitudinal interface of oxide deposit ring (40) than in the upper half of the oxide remover (30).

23. Diecasting machine according to claim 21, characterised in that a vertical longitudinal section through the concentric middle axis (m) of the oxide remover(30) in the lower half- with respect to a horizontal plane through the concentric middle axis (m) - of the oxide remover (30) exhibits a 1.1 to 1.8 times larger longitudinal interface of oxide deposit ring (40) than in the upper half of the oxide remover (30).